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: http://xmm.vilspa.esa.es/calibration/documentation/CALHB/node910.html
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Two additional extensions contain the polynomial coefficients describing the distortion for all the filters.
FILTER-U |
FILTER-B |
FILTER-V |
FILTER-UVW1 |
FILTER-UVM2 |
FILTER-UVW2 |
FILTER-WHITE |
FILTER-MAGNI |
FILTER-GRISM1 |
FILTER-GRISM2 |
FILTER-GRISM10 |
FILTER-GRISM20 |
POLYNOM_MAP |
POLYNOM_MAP2 |
Each extension FILTER-FilterId holds a binary table sampling the
detector distortion of a specific filter at a coarse resolution
( 1 datapoint every 25 subpixel).
Intermediate detector position have to be obtained by interpolation.
The distortion at the position RAWX, RAWY is described as the difference
between the measured image position on the detector and the position
assuming a linear grid. The platescale of the linear grid is the
value of the keyword PLTSCALE (units: arcsec/pixel).
The format of the binary tables follows the description of
the CCF ICD [5].
The distortion can alternatively be obtained using two polynomials
describing the offset in x- and y-direction.
Analysis of calibration data showed
that the detector distortion can be parameterized with an up to 7th order
polynomial. The 36 parameters , ...,
(cf. section 3.5.5) needed to calculate the distortion
in x- and y-direction at any detector position
(equivalent to RAWX_OFF and RAWY_OFF) are stored in the columns
XPOLYCOEF, YPOLYCOEF of the extension POLYNOM_MAP.
There is one set of coefficients per filter element in the POLYNOM_MAP
extension. The filter element is found in the column FILTER_ID.
The required binary table entry is identified by matching the
value of the column FILTER_ID with the State Variable FilterID.
The reverse transformation from distorted to flat image is available
from the polynomial stored in the extension POLYNOM_MAP2. This has
an identical format to the POLYNOM_MAP block.
The format of the AstroMet file is as follows
Binary table: FILTER-FilterID
Twelve binary extensions, one per FilterID
Binary table: POLYNOM_MAP
column name | column type | column unit | comment |
FILTER_ID | 9A | not applicable | filter ID |
PLTSCALE | E | arcsec/subpixel | platescale of filter specified in FILTER_ID |
XPOLYCOEF | 36E | not applicable | coefficients of x-distortion polynomial |
in ascending order | |||
YPOLYCOEF | 36E | not applicable | coefficients of y-distortion polynomial |
in ascending order |
Binary table: POLYNOM_MAP2
column name | column type | column unit | comment |
FILTER_ID | 9A | not applicable | filter ID |
PLTSCALE | E | arcsec/subpixel | platescale of filter specified in FILTER_ID |
XPOLYCOEF | 36E | not applicable | coefficients of x-distortion polynomial |
in ascending order | |||
YPOLYCOEF | 36E | not applicable | coefficients of y-distortion polynomial |
in ascending order |